During the fabrication of such integrated circuits as memory devices, it is conventional to test the integrated circuits at several stages during the fabrication process. For example, the integrated circuits are normally connected to a tester with a probe card when the integrated circuits are still in wafer form. In a final test occurring after the integrated circuits have been diced from the wafer and packaged, the integrated circuits are placed into sockets on a load board. The load board is then placed on a test head, typically by a robotic handler. The test head makes electrical contact with conductors on the load board, and these conductors are connected to the integrated circuits. The test head is connected through a cable to a high-speed tester so that the tester can apply signals to and receive signals from the integrated circuits.
While the above-described testing environment works well in many applications, it is not without its limitations and disadvantages. For example, it is very difficult to test various timing characteristics of the integrated circuits, particularly at the high operating speeds for which such integrated circuits are designed. This difficulty results primarily from the propagation delays in transferring signals through the cable between the tester to the test head. The cables that are typically used in such testing environments are often fairly long, thus making the propagation delays of signals coupled to and from the integrated circuits correspondingly long and often difficult to predict.
Another problem with the above-described testing environment is that it may not accurately simulate the conditions in which the integrated circuits will be actually used. In actual use, integrated circuits, such as dynamic random access memory (“DRAM”) devices, are typically mounted on a printed circuit board. Signals are applied to the integrated circuits by other integrated circuits mounted on the board, and signals generated by the integrated circuits are received by other integrated circuits mounted on the board. Most of the signals are not coupled to and from the integrated circuits through long cables coupled to distant electronic devices. Therefore, the testing environment is normally quite different from the environment in which the integrated circuits will operate in normal use.
While techniques have been developed to deal with these difficulties, the use of these techniques results in testers that are highly complex and often very expensive. A large number of testers are normally required for a high capacity semiconductor fabrication plant, thus greatly increasing the cost of the plant and the expense of testing the integrated circuits.
There is therefore a need for a testing system and method that can inexpensively test integrated circuits at high speed, and do so in an environment that accurately simulates the conditions in which the integrated circuits are likely to be used in normal operation.